1. Field of the Invention
The invention relates to numerical control systems having servotype position control means and in particular to a resolver digitizer generating a digital signal indicative of the absolute position of the member being controlled within each revolution of the resolver.
2. Description of the Prior Art
Numerical Control Systems for use in connection with the operation of various instrumentalities such as milling and drafting machines, as well as process controllers have been known to industry for many years. In a typical numerical system, position commands are generated in response to data derived from a record, such as a punched tape, and are applied as one of two inputs to a servo control loop. The servo loop serves to produce an error signal indicative of the instantaneous difference between a commanded position and the actual position of the member being controlled. The error signal is then applied to a motive means of the position control system tending to drive the error signal to zero.
The typical prior art servo mechanism error signal generator comprises a plurality of resolvers or inductrosyns one for each axis of controlled displacement, operative to produce a series of phase shifted signals, wherein the degree of the phase shift is a function of the departure of the actual position from the commanded position at a given time. The error signals are then applied to a zero crossing detector to produce a square wave having a variable phase relative to a reference signal. This phase shiftable square wave is applied to a phase shift detector which produces an output waveform representing the position error signal. The output waveform is a pulse width modulated signal, the d.c. value of which represents the position error. This d.c. signal is then applied to the input drive mechanism which changes the position of the member being controlled accordingly.
The form of the d.c. signal, as is apparent to those skilled in the art, is a voltage which varies linearly from a negative value representing a position lag to positive value representing a position lead over a range of displacement increments representing actual position error. In this type of system, the complete servo loop is external to the computer, and the computer never receives data indicative of the actual position of the member being controlled.
With the event of low cost general purpose computers having excess storage and computation capacities, the more recent art has moved in a direction towards reducing the peripheral or external hardware of the servo loop and incorporating their functions in the general purpose computer. In newer systems data indicative of the actual position of the member being moved is maintained in the computer. The error signal is then computed in the general purpose computer by comparing the actual position with the commanded position. These systems embody a resolver follower which generates digital data indicative of incremental changes in the position of the resolver. This digital data is summed in an actual position register to give the actual position.
Past experience with these types of systems has shown that this method is not totally satisfactory because errors are cumulative, and start up requires that the system be precisely set at a predetermined starting location for the actual position register to accurately contain the actual position.
The disclosed system eliminates both of these problems by providing to the computer digital data indicative of the absolute position of the resolver within each revolution of the resolver eliminating the accumulation of errors in the computer's actual position register and the need for returning the system precisely to a predetermined start location.